1. Field of the Invention
The present invention relates in general to printed circuit boards and in particular to a circuit board having embedded heaters.
2. Description of Related Art
Integrated circuits (IC) are often attached to a printed circuit board by soldering the IC leads to pads on the surface of the board. Other parts such as discrete components, IC sockets, connectors and heat sinks are also typically soldered to pads on a printed circuit board. In order to replace a part soldered to a circuit board with a new part we melt the solder attaching the old part to the circuit board so that the part can be removed, and then solder the new part in its place. However solder connecting a part to a circuit board may not be readily assessable to a heating element; the solder may be in an area between closely spaced parts, may be covered by another part such as a heat shield, or may be covered by the part itself. For example a xe2x80x9cball-gridxe2x80x9d IC package is attached to a circuit board via an array of solder balls on the underside of the package. It is possible to melt solder attaching a part to a circuit board by heating the entire part, but the heat needed to melt the solder can sometimes damage the part. It is also possible to heat solder attaching a part to the top of a circuit board by applying heat to the under side of the circuit board below the solder. However this may be difficult to do when the circuit board is thick or when the under side of the circuit board is covered with other parts, as is the case for a double-sided circuit board. It can also be difficult to melt solder holding one part to a densely packed circuit board without melting solder holding other nearby parts to the circuit board.
The operating speed of transistors in an IC implemented in complementary metal-oxide semiconductor (CMOS) technology or other semiconductor technologies is highly dependent on the operating temperature of the IC. Therefore in order to stabilize the operating speed of a CMOS or other type of IC, it has been known link the IC to a controllable heat source, to monitor the temperature of an IC or the switching speed of transistors implemented on the IC, and to control the amount of heat the source provides to the IC so as to maintain its temperature or at a constant level. However prior art systems for doing this have relied on bulky and expensive external heaters to-supply the heat to the IC.
What is needed is a heating system for selectively applying heat to solder connecting individual parts to a circuit board when the solder is not readily accessible to an external heat source. The heating system should also be able to provide heat to the parts for regulating their operating temperature.
A circuit board in accordance with the present invention includes one or more embedded heating elements for generating heat. The circuit board includes a thin upper layer of insulating material upon which parts such as integrated circuits, connectors and other devices are mounted and the heating element, residing below the upper layer and formed of material such as copper nichrome that generates substantial heat when conducting current. When a voltage pulse is applied across the heating element, a resulting current pulse passes through the heating element causing it to produce a pulse of heat.
In accordance with one aspect of the invention each heating element is positioned below a corresponding integrated circuit or other part that is soldered to the upper layer. The heat the heating element produces is used to temporarily melt solder attaching the part to the upper layer so that it can be easily removed or to temporarily melt the solder when the part is being mounted on the upper layer.
In accordance with another aspect of the invention each heating element is used as a heat source for regulating the temperature of the part mounted on the circuit board above the heating element.
In accordance with a further aspect of the invention, in one embodiment thereof, each heating element is a separate segment of a grid of heating conductors embedded under the upper layer. A voltage pulse applied across the entire grid can be used to supply a pulse of heat to all of the parts mounted on the circuit board. However a voltage pulse applied across only a particular segment of the grid produces substantial current only in that segment of the grid. In such case the grid supplies a substantial heat pulse only to those parts mounted on the circuit board above the segment of the grid that received the voltage pulse. Accordingly the grid can supply heat concurrently to all parts mounted on the circuit board or independently to particular subsets the parts.
It is accordingly an object of the invention to provide an inexpensive and convenient means,for selectively applying heat to parts mounted on a circuit board even when the parts are not readily accessible to an external heat source.
The concluding portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements.